The invention relates generally to displacement measuring devices and in particular to capacitive or electrostatic displacement transducers and associated electronic circuitry.
In the past, linear measuring devices for taking dimensions were mostly mechanical. In machine shops and factories, dimensions of relatively small parts were traditionally measured by mechanical calipers and micrometers. The introduction of dials and number-wheel or meter-type displays did not alter the fact that the display was linked mechanically to the movable member or jaw of the measuring gauge.
Recently a new breed of instrumentation has appeared, replacing the mechanical linkage with digital displays which eliminate many of the problems experienced with reading mechanical gauges. For example, battery-powered, microprocessor-based, digital-reading, handheld six inch calipers are now commercially available at prices which make them competitive with mechanical calipers. The new electronic calipers, however, have an added advantage of alternatively switching between inch and metric displays with so called "floating zero" capability as well as electronic memory. This latter feature retains a measured dimension taken in an area where the display cannot be seen, thus allowing the jaws of the caliper to be moved without losing the reading. Besides the normal measurements of outside, inside and depth dimensions taken with mechanical calipers, the floating zero feature of electronic digital calipers allows new measurement techniques offering direct reading of the variation from a nominal specified dimension with the correct sign (plus or minus).
A displacement transducer provides the input to the electronic circuitry of the gauge. Of the several transducer technologies which are evolving, optical encoders and capacitive displacement transducers are among the most popular. Capacitive transducers are particularly attractive because of their ruggedness, minimal size and low power requirements. Capacitive transducers employ an elongated array of thin, flat conductive elements arranged on parallel surfaces in spaced opposition on two relatively movable members such as the slide and scale or bar of an otherwise standard caliper. Using lithographic circuit etching techniques, the individual elements (capacitor plates) of the arrays can be made extremely small with very high precision. Moreover, by extending the scale pattern, precision measuring devices of greater length can be used to fashion height gauges, robot positioning transducers and other electronic measuring devices.
Capacitive linear displacement measuring technology is, however, far from being perfected. Some of the problems which remain have to do with optimizing the configuration of the arrays of conductive elements that form the capacitive transducer. Others have to do with designing an input signal conversion system which is not only more accurate, capable of repeatable readings to the nearest ten thousandth of an inch, but also capable of handling traverse speeds on the order of 100 inches per second or more without losing track of the slider position. In addition, these features must be achieved while consuming as little power as possible, preferably on the order of 10 to 50 microamps, to prolong battery life.
Rapid slide motion relative to the scale is a problem for capacitive displacement transducers which employ systems for digitally converting the cyclically varying input signal from the transducer as the slide moves relative to the scale. After slide motion with peak speeds in excess of 50 inches per second, prior art capacitive displacement calipers yield either incorrect readings or simply an error indication. Unfortunately, however, maximum hand slide speeds in the regime of 60 to 100 inches per second are encountered in practice particularly by skilled mechanics used to the unlimited slide speed of mechanical gauges.